Step One: Better Audio Response with the FT8900 Yaesu
MH-48A6J By Improving Acoustics
I picked up an FT8900 for my home and another for my vehicle and
have been generally pleased with them. But right from the start, I
got reports that both of them were quite weak in the transmit
audio department. If you own a Yaesu FT8900 and use the MH-48A6J
microphone, you also may have received a few reports that the
transmit audio is fairly low and has a pronounced roll-off of low
frequencies resulting in a fairly thin sound when compared to some
other radios. While this low frequency attenuation might be
desirable in certain applications, it really is tough to live with
if you prefer your audio to be more natural sounding with better
low end frequency response. Surprisingly, it appears that
Yaesu went out of its way to mechanically block the port in the
front of the mic case to help produce the restricted audio
effect. Taking the mic apart reveals that only 2 of the 3
tiny mic element holes in the front of the case are actually open,
and they are quite small, forming an audio high pass port. They
also saw fit to place the mic element in a small rubber boot that
blocks much of the front of the electret mic cartridge, which may
also reduce the frequency response of the element. I chose to work
on these mechanical restrictions to see what the result might be,
and found some improvement since opening the mic up.
Directions for the mechanical modifications of the mic are below:
1. Remove the back of the microphone case. (3 screws)
2. Lift off the PTT lever.
3. Lift off the UP/DOWN key assembly at the top of the mic.
4. Remove the 3 visible screws, one in the upper left, two
near the bottom end of the mic.
5. Remove the small screw fastening the PTT micro switch
through the board to the mic front casing.
6. Gently lift the board away, taking care to not stress
the wires to the electret element.
7. Gently work a small screw driver around the mic element
rubber boot, and slowly work it out from the mic front casing. Be
gentle and take your time. I found that there was a spot near the
top of the mic housing where I could work the screwdriver in to
get things started. Lift the mic element out of its housing when
you have it loosened up.
8. Remove the plastic baffle insert in the molded mic
housing, it should fall out easily (see picture below).
9. Using a 1/16 inch drill bit, drill out the two existing
holes in the front of the microphone and the third fake one. Then
slide the drill bit back and forth to transform the three holes
into a slot. (See the picture below for an idea of how this will
look when you are done.)
10. Carefully remove the black rubber boot from the electret
element, again, take your time because it is fairly tight.
11. Enlarge the hole through the front of the boot to open access
to more of the front of the mic element.
12. Place the black rubber boot back onto the electret element.
13. Wad up a tiny bit of cotton and place it inside the mic
housing where the baffle was. Use enough to loosely cover the
bottom of the little chamber that houses the electret. This helps
to act as a pop filter. Using too much may result in a muffled
sound, you really just need a few wisps.
14. Carefully guide the mic element back into the molded mic
housing. Make sure it is seated all way down into the cylinder.
15. Mount the circuit board inside the front of the microphone and
tighten down the screws. Then wad some cotton to fill the mic
housing. This microphone seems to have some odd acoustic
resonances from the hollow space in the back of the microphone.
The cotton will reduce this effect greatly.
Note: Remove the plastic
baffle that the arrow is pointing to.
Note: I got a C+ in shop but the slot still came out fine!
Step Two: Better Audio Response with the FT8900 Yaesu
By Increasing Coupling
The worst culprit regarding restriction of the low end audio
appears to be in the speech amplifier built into the remote
display head of the radio. A circuit diagram for the
PANEL-SUB-UNIT is located on the back of the MAIN UNIT block
diagram. One glance will reveal that they have coupled the output
of the mic through a .0056 uf capacitor to the input of the first
stage of the speech amp. A larger value would have produced much
smoother response, and much more audio. However, this circuit
board is populated with devices that are the size of a flea, and I
would not even attempt to mess with it! All is not lost though,
you can still bring about some improvement right in the microphone
unit itself. If you examine the schematic below, and trace the
audio path from the output of the electret mic element to pin 5 of
the connection plug for the mic cord, you will see why the low end
response of this microphone is so restricted. The 0.1uf capacitor
(C5003) in parallel with the 1k resistor forms a high pass circuit
that effectively blocks the passage of a good percentage of the
normal low end voice frequencies. The easiest way around this is
to simply bridge a cap from the mic output to pin 5. All the RF
bypassing and decoupling will still work effectively, but the
audio path will now bypass the 0.1uf (C5003) frequency shaping
capacitor and the 0.33uf (C5002) coupling cap. I chose a tantalum
10uf cap (RS part number 272-1436) for this task. This will boost
the low end frequency response and also raise the total audio
output of the mic slightly, which in my opinion is a good thing
because the FT8900 seems a little lean in that department with the
stock setup. Directions for completing this modification are
1. Remove the back of the microphone case. (3
2. Lift off the PTT lever.
3. Lift off the UP/DOWN key assembly at the top of
4. Locate the caps that are to be bridged. (Note in
the picture below the caps with white arrows pointing to them.)
picture below to see where the two solder connection points
are. It is important to pay attention to the polarity of the
cap as you are connecting to the voltage feedpoint of the mic
element. In the picture below, the plus lead of the cap is
soldered to the left side of the cap labeled V104 (C5003) and
the other leg is soldered to the right side of the cap labeled
V334 (C5002). I placed a chunk of double sided foam tape
behind the cap to help hold it steady when soldering and to
prevent the cap from vibrating or stressing the solder points
after the mic is put back into service. After bending the
leads of the cap to fit, tin the leads with a low wattage iron
using a very sharp tip. Then press it into place. When the cap
is in position, solder it to the pads at the ends of the
surface mount devices. You won't need much solder, and don't
linger on those tiny solder pads for too long!
confirming that the cap is securely soldered, reassemble
the mic and you are done with this part of the
Step Three: More Audio by
Increasing the Deviation of the FT8900
The final step to raising the audio level of the FT8900
involves accessing the service menu and increasing the
deviation setting. I would urge you to record your factory
settings before you adjust them, should you wish to set
the rig back to its stock setting for some reason. I ended
up setting all bands to the maximum setting in the service
menu, and this resulted in full 5 kc deviation on all
bands except for 10m, which hits the wall at approximately
3.1 kc. The procedure for deviation adjustment is below:
FT-8900 Entering the
A. Press and hold in the “Left” band (V/M) key and
the Hyper Memory #6 key while turning the radio
on. Once the radio is
powered up, release these two keys.
B. Press the following front panel keys in sequence
from left to right:
Left Side [LOW]
[V/M] [HM] [SCN] Right Side
[LOW] [V/M] [HM] [SCN]
C. You will now see “A-0 REF .xxH” on the display.
This signifies that the transceiver is now in the
FT-8900 TX Deviation
Adjustment (A-4 DEV): Note that user menu item 43 should
be set to WIDE.
A. Press the “Sub” band DIAL knob momentarily,
if needed, to switch the “Main” band to the
side. If “Main”
already appears on the “Right” side, then proceed to the
B. Tune the “Right” band frequency to 440.050
MHz, then set the Transmit Power Level to “LOW”.
C. Press and hold in the “Left” DIAL knob
momentarily to step through the A-0, A-1, A-2, A-3
alignment menus until
you arrive at the alignment parameter that reads “A-4 DEV
D. Inject a 1 kHz audio tone at a level of 50
mV from an Audio Generator. If you don't have an audio
generator, read on.
E. Press the PTT switch to activate the
transmitter and adjust the “Left” DIAL knob, as needed, so
the deviation meter
reading is 4.5 kHz. If no Audio Generator is available
adjust the deviation setting to (DEV.3FH).
F. Tune the “Right” band frequency to 146.050
MHz, then set the Transmit Power Level to “LOW”.
G. Press the PTT switch to activate the
transmitter, and adjust the “Left” DIAL knob, as needed,
the deviation meter
reading is 4.5 kHz. Again, if no Audio
Generator is available adjust the deviation setting to (DEV.3FH).
H. Press the “Left” DIAL knob momentarily to
switch the “Main” band to the “Left” side.
I. Tune the “Left” band frequency to
52.050 MHz, then set the Transmit Power Level to “LOW”.
J. Press the PTT switch to activate the
transmitter, and adjust the “Right” DIAL knob as needed,
the deviation meter reading
is 4.5 kHz. Again, if no Audio Generator is
available adjust the deviation setting to (DEV.3FH).
K. Tune the “Left” band frequency to 29.050 MHz,
then set the Transmit Power Level to “LOW”.
L. Press the PTT switch to activate the
transmitter, and adjust the “Right” DIAL knob to the
value available (DEV.3FH).
This will increase your deviation on 10 meters to just a
bit over 3 kHz.
M. Turn off the FT-8900 and then power on as you
would normally, to operate as usual.
Summary of the Audio
The end result of the acoustic improvement, heavier
coupling capacitance, and deviation adjustments, should be
a much fuller sounding transmit signal. I have been very
pleased with the audio reports I have received since
completing the changes. Reports indicate that the FT8900
sounds very natural, with a good balance of highs and lows
throughout the voice frequencies. Best of all, it is
simply much louder and easier for other stations to hear.
Audio clips in mp3 format of my FT8900 and MH48 before and
after modification are below, along with an Adobe Audition
screenshot of each of their waveforms.
microphone with front port slotted, 10uf coupling cap,
and deviation set to DEV.3FH
The total time for modifying each mic and adjusting each
of my FT8900 rigs was about 45 minutes, but then again, I
have never been accused of rushing anything, and my
experience with surface mount construction is very
limited, so chances are you will get it done faster.
Disclaimer: This modification is offered in the
spirit of Ham Radio experimentation. If you end up
damaging or destroying your MH-48A6J or any other
equipment, I am not responsible.